This invention relates to a so-called multi-digit luminescent display tube, such as a fluorescent or phosphorescent display tube, and more particularly to one comprising novel grids.
In a luminescent display tube of this type, there is a multi-unit tube, generally called a multi-digit tube, for displaying a plurality of selected displays, namely, numerals, letters, and/or symbols. The multi-unit tube comprises a plurality of luminescent anode segment groups, a hot cathode or cathodes, a first and a second support for supporting the cathode over the anode segment groups, and a plurality of grid members between the cathode and the respective anode segment groups.
The display tube of the multi-unit type is typically used in an electronic portable calculator in which a large scale integration circuit (an LSI) is frequently adapted for portability of the calculator. Since a circuit is generally operated at low electrical power and low voltage, the display tube which is directly driven by the large scale integration circuit should also luminesce at low voltage and low electrical power.
In the display tube used in the portable calculator, the cathode spends about 75% of the electrical power and, therefore, the consumption of electrical power at the cathode should be improved for low power operation of the display tube. Sophisticated display tubes were supplied with about 90 mA of electrical current per one cathode. A cathode of a recent tube has been supplied with about 11 mA. In other words, the diameter of the cathode has become thin from 31 microns to 8.1 microns. For a thin cathode, the supports have relatively become large in comparison with the cathode. Therefore, both longitudinal end portions of the cathode are excessively cooled due to heat leakage, namely, conduction of heat, at the supports. As a result, the thermal electron emission is lowered at the end portions of the cathode. Consequently, the displays of both end units are apt to be bad in comparison with those of other units.